UpdatedAug 5, 2026

Koa Error Handling

Question Variations

"Why does Koa error middleware need to be registered first?"
"When should you use `ctx.throw()`?"
"What is the purpose of Koa’s `error` event?"

Why This Is Asked

Koa’s async middleware cascade makes centralized errors straightforward, but correct placement and response safety remain essential. This question evaluates error boundaries and client-safe API design.

Key Concepts

An outer middleware wraps downstream errors with try/catch around await next().
ctx.throw() creates HTTP-aware errors for expected client failures.
Central handlers should log original errors and return safe response contracts.
app.on('error') is useful for app-level logging but does not replace response handling.

Question Variations

“Why does Koa error middleware need to be registered first?”
“When should you use ctx.throw()?”
“What is the purpose of Koa’s error event?”

Answers by Technology

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Expected Answer (Koa 3.2.1 / Node.js 18+)

Place a single error boundary first in the Koa middleware stack. It wraps await next() in try/catch, so it sees exceptions and rejected promises from all downstream middleware. For expected request failures, use ctx.throw(status, message) or throw a structured error; the boundary should log the original failure, map known operational errors to a safe response, and return a generic 500 for unknown failures.

Koa emits an application error event for logging, but that event does not replace a response policy. Do not expose stack traces, database messages, or tokens in client responses. Ensure error middleware only writes a response when headers have not already been sent, and preserve an error ID in logs for diagnosis.

Why It Matters

A predictable error boundary protects sensitive details and makes every API route return consistent failure contracts.

Code Example

import Koa, { Context, Next } from 'koa';

const app = new Koa();
app.use(async (ctx: Context, next: Next) => {
  try { await next(); }
  catch (error) {
    app.emit('error', error, ctx);
    ctx.status = (error as { status?: number }).status ?? 500;
    ctx.body = { error: ctx.status < 500 ? 'request failed' : 'internal server error' };
  }
});
app.use((ctx: Context) => { ctx.throw(400, 'invalid input'); });
app.on('error', (error) => console.error(error));
app.listen(3000);

Common Mistakes

Registering the boundary after routes: It cannot catch earlier middleware failures.
Returning raw exception messages: Internal details can reach clients.

Follow-up Questions

Why does error middleware go first? (Answer: It must wrap downstream execution.)
What does ctx.throw() provide? (Answer: An HTTP-aware error with a status code.)